I am a 30 year old male. My job requires me to sit at a desk from 9-6. While working, I often forget to drink water. One in a while, on some days, I do a few exercises after work. But most days I don't. How much water do I need to drink during the day?
The daily water requirements for a 30 years old male are in general the same as for other adults. You need to consume as much water as you lose it. A young adult loses at least about 1 liter per day -- this is the obligatory water loss, which includes the loses by urinating, invisible perspiration and breathing, but not by sweating (NAP.edu). Practically everyone sweats at least a little , so everyone probably needs to drink more than 1 liter per day.
Roughly speaking, a sedentary adult living in a temperate climate will usually need about 1.5-2 liters of water per day, including the water from beverages and foods (water, tea, milk, vegetables, fruits, cooked cereals). However, even in moderate climate, on a hot summer day, with few hours of a simple exercise like walking, you will probably sweat a lot, so you might need 4 to 5 liters or more water.
Since it is not practical to measure the total water intakes and loses, you can just take care that you are well hydrated, which means you are not dehydrated or overhydrated. The main symptoms of dehydration are thirst, dry mouth, bright yellow or dark urine, increased heart rate and fatigue. When you drink more than you need, you will excrete the excessive water, so you will urinate frequently, probably more than 6 times per day.
Here is a detailed article about how much to drink per day, with all the reasoning, tests and references.
Have a bottle of water at your desk - it helps you not forget to drink.